Cloning and expression of a cDNA encoding betanidin 5-O-glucosyltransferase, a betanidin- and flavonoid-specific enzyme with high homology to inducible glucosyltransferases from the Solanaceae

Based on protein sequence data and RT-PCR, a full length cDNA encoding betanidin 5-O-glucosyltransferase (5-GT) was obtained from a cDNA library of Dorotheanthus bellidiformis (Burm,f.) N.E.Br. (Aizoaceae). 5-GT catalyses the transfer of glucose from UDP-glucose to the 5-hydroxyl group of the chromogenic betanidin. Betanidin and its conjugates, referred to as betacyanins, are characteristic fruit and flower pigments in most members of the Caryophyllales, which fail to synthesise anthocyanins. The 5-GT cDNA displayed homology to previously published glucosyltransferase sequences and exhibited high identity to sequences of several inducible glucosyltransferases of tobacco and tomato (Solanaceae). The open reading frame encodes a polypeptide of 489 amino acids with a calculated molecular mass of 55.24kDa. The corresponding cDNA was expressed in Escherichia coli. The recombinant protein displayed identical substrate specificity compared to the native enzyme purified from D. bellidiformis cell suspension cultures. In addition to the natural substrate betanidin, ortho-dihydroxylated flavonols and flavones were glycosylated preferentially at the B-ring 4'-hydroxyl group. 5-GT is the first enzyme of betalain biosynthesis in plants, of which the corresponding cDNA has been cloned and expressed. The results are discussed in relation to molecular evolution of plant glucosyltransferases.